Fix typo for mersi2/abi/ahi using bidirection instead of bidirectional

[djhoese]

standard_name for ABI, AHI, MERSI-2 L1B I had a typo of toa_bidirection_reflectance when it should be toa_bidirectional_reflectance (note the al). This fixes this typo.

• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[coveralls]

Coverage remained the same at 81.842% when pulling 2470d07 on djhoese:bugfix-bidirection-typo into 7fe50a7 on pytroll:master.

[codecov]

Codecov Report

Merging #770 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #770   +/-   ##
=======================================
  Coverage   81.84%   81.84%           
=======================================
  Files         158      158           
  Lines       22728    22728           
=======================================
  Hits        18601    18601           
  Misses       4127     4127

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[mraspaud]

LGTM